Pressure amplifiers of this type are relatively simple compared with devices for raising the pressure directly from zero to the required high value, but for many applications it is a disturbing fact that the oscillating motion of the amplifier piston system gives rise to fluctuations of the high pressure as delivered from the amplifier. Of course, the high pressure fluid is delivered through a check valve which will prevent backflow to the low or medium pressure system, but if there is a certain fluid consumption or leakage in the high pressure system a pressure drop may well occur therein if the high pressurized fluid is not fully constantly supplied from the pressure amplifier. Since the amplification is a matter of reciprocating a piston system it will be unavoidable that such pressure drops may occur, already because the piston system has to stop its action whenever its direction of movement is being inverted.
There is distinguished between single-acting and double-acting pressure amplifiers, just as with piston pumps, and obviously a double-acting device will be better suited to produce an almost constant discharge pressure, because in a double-acting system there is no operational pause connected with an idle return stroke of the piston system as in a single-acting system. However, there may still be problems with respect to a rapid and safe actuation of the means for cylically changing the direction of movement of the piston system. For many applications it is a major demand that the pressure amplifier should be of a small size and of a low price, and for that reason the single-acting devices are preferred for many purposes, e.g. for incorporation in various types of tools and vehicles. When used e.g. in connection with hydraulic actuators for the holding of workpieces being worked the pressure amplifiers should be able to maintain the discharge pressure almost constant.
Still, however, the single-acting amplifiers show the drawback of a relatively low effective working frequency due to the need of an idle return stroke for each working stroke. This implies relatively large outer dimensions for a given rate of high pressure flow, and pressure dives are liable to occur at the high pressure side.
The U.S. Pat. No. 3,737,254 discloses such an amplifier and asserts that the return switching is effected instantaneously, but since during the switching phase the axial forces acting upon the switch controlling mechanism are directed opposite the moving direction of the piston system, and since the fluid from the low pressure cylinder chambers has to pass through relatively long and narrow channels, the resulting working frequency is bound to be relatively low anyway.
For simplicity the piston system should be reciprocated by the medium pressure fluid, i.e. by the fluid moving the system through both its working strokes and its return strokes, and also the associated switch control should be effected by the medium pressure fluid. This invites to the use of a simple slide controller, which is reversed, directly or indirectly, in response to the piston system arriving at its respective extreme positions, so as to thereby be operated to connect the relevant piston chambers with the medium pressure source and a low pressure return system in an alternating manner. However, such simple control systems are well known to present some important operational problems, inter alia because, for a really safe switching operation, they have to be provided with some narrow channels, so-called nozzles, whereby the system is vulnerable to a dirt blocking and is rather slow in its reactions. Moreover the nozzles are subjected to wear, whereby after some time the system may become unstable. These problems could be reduced with the use of thicker valve channels, but that would give rise to problems with respect to the safety of the switching operations and partly unexplainable situations of equilibrium may occur, where the reciprocating piston system just stops working.
It is known to stabilize the operation based on a mechanical snap locking of the slide controller in its respective extreme positions, but this implies increased costs and space requirements, and moreover the locking system will be subjected to wear.